The present invention relates to a new and improved construction of a maintenance device for at least partially automatic cleaning and calibration of a probe containing a measured value transmitter.
In its more specific aspects, the present invention particularly relates to a new and improved construction of a maintenance device for at least partially automatic cleaning and calibration of a probe containing a measured value transmitter and which probe containing the measured value transmitter is part of a measuring device for continuously monitoring biological or chemical processes. Such processes are continuously monitored by measuring analytical parameters in a fluid medium. The probe contains a sensor which is removable from the fluid medium to be investigated, and a guide device for holding and guiding the sensor of the probe which contains the measured value transmitter, between an operative position and a maintenance position. The probe further contains a shut-off device for shutting off the probe from the fluid medium to be investigated.
In general, the aforementioned probes contain, for example, electrochemical sensors, such as pH electrodes, particularly single-rod electrode assemblies, and are used for monitoring biological or chemical processes, particularly fermentation processes. It is disadvantageous here that, in practice, the aforementioned sensors only have a limited service life and their indication is subject to change in the course of time, namely for the following reasons:
1. contamination by the reaction medium; PA0 2. zero-point drift; PA0 3. loss of gradient in the case of pH electrodes, shift of the reference voltage in the case of pH and redox electrodes, and deactivation in the case of redox electrodes; and PA0 4. total failure of the electrode. PA0 1.1 discontinuous chemical cleaning using agents which clean the sensor membrane; PA0 1.2 continuous cleaning using water which is sprayed from rinsing nozzles against the sensor or electrode membrane; PA0 1.3 mechanical continuous or discontinuous cleaning using rotating brushes or ultrasound.
The interferences caused thereby hitherto have rendered more difficult and partially impossible the continuous monitoring of biological and chemical processes because they lead to a considerable falsification of the measured values and the frequently necessary removal and replacement of the sensor brings about an interruption of the process course.
In order to eliminate the interferences arising due to the contamination mentioned as reason 1. hereinbefore, continuous or discontinuous cleaning methods have been proposed or used in the past. These include:
The aforementioned cleaning methods have several disadvantages. Thus, for example, the pH value is highly falsified using the cleaning method mentioned hereinbefore as method 1.1, and the cleaning agent generally is diluted by the fluid medium. The consumption of cleaning agent also is relatively high. The cleaning method mentioned hereinbefore as method 1.2, generally only works for loosely adhering contaminants which is why this method can only be used to a limited extent. In addition, the pH value is also falsified in this case. The cleaning method mentioned hereinbefore as method 1.3, is associated with great mechanical complexity and, in the case of abrasive media, with high wear of the moving parts. Ultrasound cleaning requires a relatively large constructional volume and, in addition, cannot be used in areas with an explosion hazard due to the high energy output. Besides the disadvantage that the aforementioned cleaning methods require relatively large constructional volumes and are not suitable for small or restricted installation dimensions, there is added the further disadvantage that their effectiveness is not always guaranteed at high pressures.
The elimination of errors which are caused in the course of time by the reasons mentioned hereinbefore as reasons 2. and 3., requires regularly repeated cleaning and calibration operations using at least two calibration solutions and corrections at the measuring instruments. The operations necessary for the elimination of these errors require a time expenditure of a few hours to several weeks, depending on the type of fluid medium to be investigated, the type of the contaminants, and the magnitude of the occurring electrode drift. However, this means that these operations result in considerable impairment of the process to be monitored and thus causes a considerable increase in costs.
It is known from German Patent Publication No. 2,557,542, published June 23, 1977, that a measuring electrode may be installed into a ball valve which separates the measuring electrode from the medium to be investigated. The ball valve position is changed in order to make the measuring electrode accessible for cleaning or calibration solutions. Furthermore, there is known from German Patent Publication No. 2,712,159, published Sept. 28, 1978, that the fluid medium to be investigated and calibration solutions can be sprayed onto the measuring electrode in small quantities through fine nozzles. Furthermore, from German Patent Publication No. 2,514,193, published Jan. 2, 1976, there is known an instrument for automatic analysis of liquid samples in which instrument test tubes containing the samples are arranged on a conveyor belt which is subdivided into sectors. By means of such conveyor belt, the test tubes are conveyed to individual analysis stations in accordance with a predetermined program. The control here preferably is effected by means of a pneumatic programming mechanism. Finally, it has been proposed that the electrode membrane be accommodated in a recess in which the electrode can be cleaned or calibrated when necessary.
These aforementioned methods are afflicted with various deficiencies, especially as they are obviously not universally applicable. Thus, for example, the arrangement according to German Patent No. 2,557,542 is only suitable for tubes. In the case of automation, a bypass would have to be installed. In addition, no information is given regarding the means and the manner in which the infeed of cleaning and calibration solutions and their metering is accomplished. In addition, during use of these methods, the temperature of the calibration solutions used is not taken into account, which leads to an impairment in the calibration accuracy which cannot be neglected.